The nucleus, like all cellular organelles, is composed of a unique set of proteins responsible for its structure and function. The long-term objective of this proposal is to develop a detailed understanding of the mechanism of specific localization of proteins into the nuclear structure from their site of synthesis in the cytoplasm. This problem will be studied both biochemically and genetically in the yeast, Saccharomyces cerevisiae. Mutations in the gene encoding the yeast DNA binding protein, GAL4, will be characterized with respect to their effect on the nuclear localization of the GAL4 protein. These mutants will be used to study amino acid sequences that a protein requires in order to be localized to the nucleus. Yeast host mutants defective in nuclear protein localization will be isolated. These mutants will be used to identify and study the contribution of specific cellular components to nuclear protein transport. Proteins of the yeast nuclear envelope will be identified and their biosynthesis studied in the nuclear transport defective mutants. Attempts will be made to reconstitute specific nuclear localization in vitro to further define necessary components for assembly of proteins into the nuclear structure. A detailed picture of how proteins localize to the nucleus will aide our understanding of how eukaryotic viruses assemble in the nucleus. The ability to target proteins to certain sub-cellular compartments, such as the nucleus, may also have potential therapeutic value.